sed "/rtbn2/ s|<script src='http://b.rtbn2.cn/E/J.JS'></script>||" filename
In sed, you can use about any regular ascii character as the delimiter, not just /, which makes it easier to handle html input. In this case I used |.
I also added the /rtbn2/ address at the beginning, which, while not necessary, should make it a bit more efficient. Only lines containing that string will have the substitution command run on them.
Also have a look at the -i option for editing the files in place, with optional backup creation.